Translucent materials such as ceramics and glasses, are prone to cracking from a variety of stress sources throughout their manufacturing cycle. Detecting cracks, voids and other inhomogeneities in or on a homogenous surface early in the production process, for example of electrical and/or optical components, beneficially avoids detecting a defect later in the production process and losses from the accumulated costs associated with processing an initially defective substrate. Later in the production cycle, pattern recognition methods as known in the art may be applied to a substrate having been processed to have a pattern on or in the surface of the substrate, which are in contrast to the present inventive methods that do not involve pattern recognition techniques and are applicable to otherwise homogenous (i.e. non-patterned) substrates or portions of a substrate's surface not comprising a pattern. It is well known that the application of pattern recognition inspection methods to a non-patterned substrate or non-patterned area of a substrate are fundamentally destined to fail, as there is no pattern to compare to a known-good or expected pattern. Furthermore, the present inventive methods are not applicable to patterned areas of a substrate as the pattern itself would be identified as an inhomogeneity in the surface of the substrate and perhaps falsely indicate a defect, or as well the pattern itself could mask or cover-up a defect in the underlying substrate, which is an object of the present invention to detect.
As defined herein, translucent materials are materials wherein at least a portion of light from an illumination source penetrates the surface of the material. By this definition, translucent materials include transparent and semi-transparent materials, such as glasses, ceramics and combinations thereof. Stresses in these materials can arise from; an inhomogeneous density distribution in the green body during ceramic forming and sintering, component machining and handling, and thermal stresses developed during sudden temperature changes, all of which can result in crack initiation in the material. For piezoelectric ceramic components, electromechanical stresses can also be created during a hot poling process, which can lead to crack formation. Unlike cracks created by forming operations and thermal-shock, cracks generated during machining, handling and hot poling are small and difficult to detect. These cracks can have a crack opening width of less than 1 μm, are typically found near the edges of a component, and can be of concern when their length exceeds about 25 μm. The presence of cracks, defects and other inhomogeneities in glass, ceramic and glass-ceramic substrates, is a potential source of unreliable functionality. Thus routine yet robust inspection methods are needed for crack detection and characterization of non-patterned (i.e. unpatterned) substrates (i.e. or non-patterned portion thereof).
The present invention provides methods and apparatus for inspecting, detecting and characterizing cracks, defects and other inhomogeneities as may be found in the otherwise homogeneous surface (i.e. non-patterned) of glass, ceramic and glass-ceramic materials, that are translucent to an illumination source.